Update readme

[Tom-Newton]

There are a list of packages mentioned in step 1 of the sunflower-toolchain readme. These seem to be required to build the cross compiler but they are not mentioned in the readme on this repo.

I think it would also be useful to add instructions for building the simulator itself, including how to add it to path so the command sf can be used from anywhere. Currently the install instructions only cover building the cross-compilers.